Interpretive Summary: Consumption of cruciferous vegetables, such as broccoli, was found to be associated with decreased risk of cancers such as prostate cancer. However, the mechanisms of action remain largely unclear. In the present studies, we utilized prostate cancer cell culture models to elucidate the mechanisms of action of two broccoli-derived phytochemicals: 3,3’-diindolylmethane (DIM) and indole-3-carbinol (I3C). We found that when present at low concentrations in the cell culture, DIM and I3C inhibited the activity of sex hormone-mediated pathways and increased metabolic pathways involved in detoxification. We also demonstrated for the first time that DIM and I3C inhibited cell growth through the production of insulin-like growth factor-1 receptor. More importantly, we showed that the two compounds differed in their way of acting and in how well they affected activities of the pathways studied. Furthermore, we showed that several other diet-derived, cancer-protective compounds similar in chemical structure to DIM and I3C exhibited many effects on cell growth and function based on the amount present in the cell growth media. However, given the concentrations of the compounds tested in the cell media, it is likely that some cellular pathways are not affected by the amounts of DIM or I3C consumed in the diet. A hypothetical model for cancer protective effects of DIM and I3C was proposed based on our results. These results provide mechanistic information on how these compounds contribute to potential anti-prostate cancer activities. This work provides new information for cancer research scientists regarding mechanisms of action of broccoli-derived and other food-derived phytochemicals, and as a basis for cancer preventive strategies that include foods rich in these compounds.

Technical Abstract:
In the present studies, we utilized prostate cancer cell culture models to elucidate the mechanisms of action of broccoli-derived phytochemicals 3, 3’-diindolylmethane (DIM) and indole-3-carbinol (I3C). We found DIM and I3C at 1-5 uM inhibited androgen and estrogen-mediated pathways and induced a xenobiotic metabolism pathway. By contrast, DIM and I3C induced cyclin inhibitors, indicators of stress/DNA damage, only at >25 uM. We also demonstrated that an inhibitory effect of DIM and I3C on cell growth involved inhibition of insulin-like growth factor-1 receptor expression. More importantly, we showed that differences in efficacies and mechanisms existed between DIM and I3C. These included differences in effective concentrations, a differential effect on androgen receptor binding, and a differential effect on a xenobiotic metabolic pathway through aryl hydrocarbon receptor-dependent and independent mechanisms. Furthermore, we determined that several other diet-derived cancer protective compounds similar to DIM and I3C exhibited pleiotrophic effects on signaling pathways that included proliferation, cell cycle, and nuclear receptors-mediate pathways. However, the efficacies and mechanisms of these compounds vary. We also showed that some cellular pathways are not likely affected by DIM or I3C when the actual concentration consumed in the diet is considered. A hypothetical model for cancer protective effects of DIM and I3C was proposed based on our results.